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We propose and demonstrate a novel fiber-optic refractive index (RI) sensor using dielectric multilayer thin films (DMFs) deposited on an optical fiber end surface, which rejects the incident light with particular wavelength. We experimentally show that the center frequency of the reflected light spectrum changes in proportion to the RI of six kinds of liquids attached to the fiber end. Its coefficient of + 0.61 nm/RIU (refractive index unit) agrees well with our simulation results based on characteristic-matrix technique. We also show that this sensor is applicable to the RI measurement of solids.